Tissue resurfacing

ABSTRACT

A method for skin treatment comprises the steps of delivering at least one pulse of radio frequency power to at least one electrode in order to create an electric field; passing gas through the electric field in order to form plasma from the gas; and applying the plasma to the surface of skin. The amount of radio frequency power may be relatively low such that the application of plasma causes denaturation of collagen within the collagen-containing tissue beneath the skin surface, which may promote the generation of new collagen within the collagen-containing tissue.

RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. patent applicationSer. No 10/792,765 filed Mar. 5 2004, which in turn is aContinuation-in-Part of U.S. patent application Ser. No. 09/789,550 (nowU.S. Pat. No. 6,723,091), filed Feb. 22, 2001, which in turn claims thebenefit of priority of Provisional Application No. 60/183,785, filedFeb. 22, 2000.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to tissue resurfacing, for example, skinresurfacing, or the resurfacing or removal of tissue located within,e.g., the alimentary canal, respiratory tracts, blood vessels, uterus orurethra.

2. Description of Related Art

Human skin has two principal layers: the epidermis, which is the outerlayer and typically has a thickness of around 120μ in the region of theface, and the dermis which is typically 20-30 times thicker than theepidermis, and contains hair follicles, sebaceous glands, nerve endingsand fine blood capillaries. By volume the dermis is made uppredominantly of the protein collagen.

A common aim of many cosmetic surgical procedures is to improve theappearance of a patient's skin. For example, a desirable clinical effectin the field of cosmetic surgery is to provide an improvement in thetexture of ageing skin and to give it a more youthful appearance. Theseeffects can be achieved by the removal of a part or all of theepidermis, and on occasions part of the dermis, causing the growth of anew epidermis having the desired properties. Additionally skinfrequently contains scar tissue, the appearance of which is consideredby some people to be detrimental to their attractiveness. The skinstructure which gives rise to scar tissue is typically formed in thedermis. By removing the epidermis in a selected region and resculptingthe scar tissue in the dermis it is possible to improve the appearanceof certain types of scars, such as for example scars left by acne. Theprocess of removing epidermal and possibly dermal tissue is known asskin resurfacing or dermabrasion.

One known technique for achieving skin resurfacing includes themechanical removal of tissue by means of an abrasive wheel, for example.Another technique is known as a chemical peel, and involves theapplication of a corrosive chemical to the surface of the epidermis, toremove epidermal, and possibly dermal skin cells. Yet a furthertechnique is laser resurfacing of the skin. Lasers are used to deliver acontrolled amount of energy to the epidermis. This energy is absorbed bythe epidermis causing necrosis of epidermal cells. Necrosis can occureither as a result of the energy absorption causing the temperature ofthe water in the cells to increase to a level at which the cells die, oralternatively, depending upon the frequency of the laser light employed,the energy may be absorbed by molecules within the cells of theepidermis in a manner which results in their dissociation. Thismolecular dissociation kills the cells, and as a side effect also givesrise to an increase in temperature of the skin.

Typically during laser resurfacing a laser beam is directed at a giventreatment area of skin for a short period of time (typically less thanone millisecond). This can be achieved either by pulsing the laser or bymoving the laser continuously and sufficiently quickly that the beam isonly incident upon a given area of skin for a predetermined period oftime. A number of passes be may made over the skin surface, and deadskin debris is usually wiped from the skin between passes. Laserscurrently employed for dermabrasion include a CO₂ laser, and anErbium-YAG laser. The mechanisms by which energy is absorbed by thetissue causing it to die, and the resultant clinical effects obtained,such as the depth of tissue necrosis and the magnitude of the thermalmargin (i.e. the region surrounding the treated area that undergoestissue modification as a result of absorbing heat) vary from one lasertype to another. Essentially, however, the varying treatments providedby these lasers may be considered as a single type of treatment methodin which a laser is used to impart energy to kill some or part of theepidermis (and depending upon the objective of the treatment, possiblypart of the dermis), with the objective of creating growth of a newepidermis having an improved appearance, and also possibly thestimulation of new collagen growth in the dermis.

Other prior art references of background interest to the presentinvention include U.S. Pat. No. 3,699,967 (Anderson), U.S. 3,903,891(Brayshaw), U.S. 4,040,426 (Morrison), U.S. Pat. No. 5,669,904,WO95/0759, WO95/26686 and WO98/35618.

SUMMARY OF THE INVENTION

The present invention provides an alternative to known skin resurfacingtechniques, apparatus and methods of operating such apparatus.

According to a first aspect of the present invention, a tissueresurfacing system comprises: a surgical instrument having a gas conduitterminating in a plasma exit nozzle, and an electrode associated withthe conduit, and a radio frequency power generator coupled to theinstrument electrode and arranged to deliver radio frequency power tothe electrode in single or series of treatment pulses for creating aplasma from gas fed through the conduit, the pulses having durations inthe range of from 2 ms to 100 ms.

The application of an electric field to the gas in order to create theplasma may take place at any suitable frequency, including theapplication of standard electrosurgical frequencies in the region of 500kHz or the use of microwave frequencies in the region of 2450 MHz, thelatter having the advantage that voltages suitable for obtaining theplasma are more easily obtained in a complete structure. The plasma maybe initiated or “struck” at one frequency, whereupon optimum powertransfer into the plasma may then take place at a different frequency.

In one embodiment a radio frequency oscillating voltage is applied tothe electrode in order to create a correspondingly oscillating electricfield, and the power transferred to the plasma is controlled bymonitoring the power reflected from the electrode (this providing anindication of the fraction of the power output from the power outputdevice which has been transferred into the plasma), and adjusting thefrequency of the oscillating voltage from the generator accordingly. Asthe frequency of the oscillating output from the generator approachesthe resonant frequency of the electrode (which is affected by thepresence of the plasma), the power transferred to the plasma increases,and vice versa.

Preferably, in this embodiment, a dipole electric field is applied tothe gas between a pair of electrodes on the instrument which areconnected to opposing output terminals of the power output device.

In an alternative aspect of the invention a DC electric field isapplied, and power is delivered into the plasma from the DC field.

The gas employed is preferably non-toxic, and more preferably readilybiocompatible to enable its natural secretion or expulsion from the bodyof the patient. Carbon dioxide is one preferred gas, since the humanbody automatically removes carbon dioxide from the bloodstream duringrespiration. Additionally, a plasma created from carbon dioxide ishotter (albeit more difficult to create) than a plasma from, for exampleargon, and carbon dioxide is readily available in most operatingtheatres. Nitrogen or even air may also be used.

According to another aspect of the invention, a gas plasma tissueresurfacing instrument comprises: an elongate gas conduit extending froma gas inlet to an outlet nozzle and having a heat resistant dielectricwall; a first electrode located inside the conduit; a second electrodelocated on or adjacent an outer surface of the dielectric wall inregistry with the first electrode; and an electrically conductiveelectric field focussing element located inside the conduit and betweenthe first and second electrodes.

Some preferred features are set out in the accompanying dependentclaims. The system described hereinafter has the benefit of being ableto produce rapid treatment at the tissue surface while minimisingunwanted effects, e.g. thermal effects, at a greater than requireddepth.

A further aspect of the present invention provides a method of skinresurfacing at least the epidermis of a patient using a surgical systemcomprising an instrument having an electrode connected to a power outputdevice, the method comprising the steps of: operating the power outputdevice to create an electric field in the region of the electrode;directing a flow of gas through the electric field, and generating, byvirtue of the interaction of the electric field with the gas, a plasma;controlling power transferred into the plasma from the electric field;directing the plasma onto the tissue for a predetermined period of time,and vaporising at least a part of the epidermis as a result of the heatdelivered to the epidermis from the plasma.

The invention also provides, according to a further aspect, a tissueresurfacing system comprising: a plasma treatment instrument having agas conduit terminating in a plasma exit nozzle, and an electrodeassociated with the conduit, and a radio frequency power generatorcoupled to the instrument electrode and arranged to deliver radiofrequency power to the electrode in a single or series of treatmentpulses each comprising a burst of radio frequency oscillations, thegenerator including a controller which operates to control the width ofthe treatment pulses to a predetermined width. The controller ispreferably arranged to adjust the treatment pulse width by generatingcorresponding control pulses which are fed to a radio frequency powerstage of the generator to alter the level of the power stage output froma substantially quiescent level to a predetermined, preferably constant,output power level for time periods each equal to a demanded pulsewidth, whereby a gas plasma is produced for such time periods. The timeperiods and/or the power level may be adjusted by the controller toyield metered treatment pulses for the instrument each having apredetermined total energy content.

It is possible, within the scope of the invention, for the radiofrequency power output to be modulated (100% modulation or less) withineach treatment pulse.

Treatment pulse widths of from 2 ms to 100 ms are contemplated, and arepreferably within the range of from 3 ms to 50 ms or, more preferably,from 4 ms to 30 ms. In the case where they are delivered in series, thetreatment pulses may have a repetition rate of 0.5 Hz to 10 Hz or 15 Hz,preferably 1 Hz to 6 Hz.

From an instrument aspect, the invention also provides a gas plasmatissue resurfacing instrument comprising an elongate gas conduitextending from a gas inlet to a plasma exit nozzle, at least a pair ofmutually adjacent electrodes for striking a plasma from gas within theconduit, and, between the electrodes, a solid dielectric wall formedfrom a material having a relative dielectric constant greater than unity(preferably of the order of 5 or higher). Advantageously the conduit isformed at least in part as a dielectric tube of such material, theelectrode comprising an inner electrode inside the tube and a coaxialouter electrode surrounding the tube.

Other aspects of the invention include the following:

A method of operating a surgical system is provided comprising a poweroutput device which generates an output signal at an output terminal, acontroller capable of receiving input signals from a user andcontrolling the power output device accordingly, an instrument having atleast one electrode connected to the generator output terminal via afeed structure, a supply of gas and a further feed structure forconveying the gas from the supply to the instrument, the methodcomprising the steps of receiving input signals from a user, andoperating the controller to determine from the user input signals amanner in which the power output device is to be controlled; operatingthe power output device to supply a voltage to the at least oneelectrode, thereby to create an electric field in the region of theelectrode; passing gas through the electric field, and creating byvirtue of the intensity of the electric field a plasma from the gas; andcontrolling, in accordance with the user input signals to thecontroller, the power output device to control the power delivered intothe plasma. The controller may operate to control the power outputdevice to deliver a predetermined level of energy into the plasma, andthe controller may further control the rate of flow of gas through theelectric field.

The gas preferably comprises molecules having at least two atoms.

There is also provided a surgical system for use in tissue resurfacingcomprising: a user interface which receives input signals from a userrelating to desired performance of the system; a power output devicewhich generates a voltage output signal at an output terminal; a gassupply, an instrument having an electrode connected to the outputterminal of the power output device thereby to enable the generation ofan electric field in the region of the electrode when the power outputdevice is operated to produce an output voltage at the output terminal,the instrument additionally being connected to the gas supply andfurther comprising a conduit for passing gas from the supply through theelectric field in the region of the electrode to create a plasma; and acontroller which is connected to the user interface and the power outputdevice, the controller being adapted to receive and process signals fromthe user interface and to control, on the basis of the user interfacesignals, the delivery of power from the power output device into theplasma The controller may be additionally adapted to control the timeperiod over which power is delivered into the plasma

User interface signals from the user interface to the controller mayrelate to a total amount of energy to be delivered into the plasma. Thesystem may further comprise a gas flow regulator connected to thecontroller, the controller being additionally adapted to control to arate of flow of gas from the supply. The controller may receive feedbacksignals indicative of power delivered to the plasma.

The power output device may include a tuneable oscillator, and thecontroller being connected to the oscillator to tune the oscillator onthe basis of feedback signals indicative of power attenuated within theinstrument. Typically, the output frequency of the oscillator lieswithin the band of 2400-2500 MHz.

A method is provided for operating a surgical system comprising a poweroutput device which produces an oscillating electrical output signalacross a pair of output terminals, an instrument having a pair ofelectrodes each of which is connected to one of the output terminals ofthe power output device, a controller which receives input signals froma user interface and controls the power output device accordingly, and asupply of gas connected to the instrument, wherein the method comprisesthe steps of: operating the power output device to apply an oscillatingvoltage across the electrodes of the instrument, thereby to create anelectric field in the region of the electrodes; passing gas through theelectric field and striking a plasma between the electrodes of theinstrument; and operating the controller to control the power deliveredinto the plasma from the power output device.

A surgical system is provided comprising: a power output device whichgenerates a radio frequency oscillating output signal across a pair ofoutput terminals; an instrument having a first pair of electrodesconnected to respective output terminals of the power output device andwhich are part of a first resonant assembly which is resonant at apredetermined frequency, and a second pair of electrodes connected torespective output terminals of the power output device and which arepart of a second resonant assembly which is also resonant at thepredetermined frequency, a gas supply which supplies gas to theoscillating electric field between the first pair of electrodes and tothe oscillating electric field between the second pair of electrodes;wherein the first resonant assembly is resonant at the predeterminedfrequency prior to formation of a plasma from the gas, and the secondresonant assembly is resonant at the predetermined frequency subsequentto the generation of a plasma. In such a system the first pair ofelectrodes may comprise an inner electrode and an outer electrodeextending substantially coaxially with, and around the inner electrode,and the second pair of electrodes may comprise a further inner electrodeand said outer electrode. The system may operate such that, duringresonance of the first resonant structure, a potential difference iscreated between the inner electrode and the further inner electrode, anda plasma is initially struck between the inner electrode and the furtherinner electrode as a result of the potential difference.

A further aspect of the invention includes a surgical system comprising:a power output device which generates a radio frequency oscillatingoutput signal across a pair of output terminals; an instrument having apair of electrodes connected to respective output terminals of the poweroutput device via a feed structure, to create an oscillating electricfield between the electrodes; a gas supply and a conduit from the gassupply to the electric field, to enable gas passing through the electricfield to be converted into a plasma and to pass out of an aperture inthe instrument; wherein the instrument comprises a voltagetransformation assembly providing step up of the voltage output from thepower output device, and supplying the stepped-up voltage across theelectrodes thereby to intensify the electric field between theelectrodes. In such a system the voltage transformation assembly maycomprise a structure within the instrument having a resonant frequencywithin the radio frequency oscillating output bandwidth. The resonantstructure may comprise at least one length of transmission line havingan electrical length equal to one quarter of a wavelength of theoscillating output signal of the power output device.

Another aspect of the invention provides a surgical instrumentcomprising: a pair of electrodes; a connector connectible to a feedstructure, thereby to enable a signal from a generator to be conveyed tothe electrodes; at least a first section of transmission lineelectrically connected to the electrodes and to the feed structure, thesection of transmission line having an electrical length substantiallyequal to one quarter of a wavelength of an electromagnetic wave having afrequency in the range 2400 MHz to 2500 MHz. This instrument may furthercomprising a second section of transmission line electrically connectedto the connector and to the first section of transmission line, thefurther section of transmission line having an electrical lengthsubstantially equal to the length of the first section of transmissionline, wherein the characteristic impedances of the first and secondsections of transmission line are different, the first and secondsections of transmission line forming an impedance matching assemblybetween a relatively low characteristic impedance of a feed structurewhich is connectable to the instrument via the connector and arelatively high impedance electrical load provided by a plasma formedbetween the electrodes.

There is also provided a surgical instrument comprising: a pair ofelectrodes separated from each other; a connector for connecting anelectrical signal from a feed structure to the electrodes thereby toenable the creation of an electric field between the electrodes; a gasinlet port; a gas conduit for conveying gas from the inlet port to theelectrodes thereby to allow gas to pass between the electrodes to enablethe creation of a plasma between the electrodes when an electric fieldis applied between them; and an aperture in the instrument through whichplasma may be expelled under pressure of gas passing is along the gasconduit. In such an instrument, gas pressure within the conduit mayforce plasma out of the aperture in a first direction, and theelectrodes may be spaced apart at least in the first direction.

Yet a further aspect includes a surgical instrument comprising: aconnector having a pair of electrical terminals; first pair ofelectrodes provided by an inner electrode and an outer electrodeextending coaxially around the inner electrode; a second pair ofelectrodes provided by a further inner electrode and said outerelectrode, the first and second pairs of electrodes being electricallyconnectable via the connector to a generator to enable creation of anelectric field between the inner and outer electrodes and the furtherinner and outer electrodes respectively, a gas inlet port, and a conduitfor conveying gas from the inlet port through the electric field therebyto enable the formation of a plasma from the gas; the first pair ofelectrodes forming at least a part of a first resonant assembly, and thesecond pair of electrodes forming at least a part of a second resonantassembly, the first and second resonant assemblies being resonant at adifferent frequencies prior to the formation of a plasma, thereby toenable, prior to the formation of a plasma, the creation of an electricfield between the inner and further inner electrodes which may be usedto strike a plasma.

There is also provided a method of operating a surgical instrumenthaving first and second pairs of electrodes, the electrodes of each pairbeing connected to different output terminals of a power output devicewhich generates an oscillating electrical output signal, the methodcomprising the steps of: operating the power output device to apply anoscillating electrical signal to the first and second pairs ofelectrodes; causing resonance of resonant assembly of which the firstpair of electrodes form at least a part; creating, by virtue of theresonance, a potential difference and thus an electric field between anelectrode of the first pair of electrodes and an electrode of the secondpair of electrodes; passing a gas through the electric field and, byvirtue of interaction between the electric field and the gas, forming aplasma The electrodes between which the electric field is created mayboth be connected to the same output terminal of the power outputdevice. Generally, the formation of a plasma results in a change ofelectrical characteristics of the second pair of electrodes such thatthey are at least a part of a further resonant assembly which isresonant at the frequency of the oscillating electrical output signal,the method then further comprising the step, subsequent to the formationof a plasma, of causing resonance of the further resonant assembly tocreate an electric field of sufficient intensity between the second pairof electrodes to maintain the plasma, and delivering power into theplasma from the oscillating output signal.

Yet another aspect of the invention is a method of operating a surgicalinstrument having first and second pairs of electrodes, the electrodesof each pair being connected to different output terminals of a poweroutput device which generates an oscillating electrical output signal,the method comprising the steps of: operating the power output device toapply an oscillating electrical signal to the first pair of electrodes;applying the oscillating electrical output signal to the first pair ofelectrodes; causing resonance of a first resonant assembly of which thefirst pair of electrodes forms a part, and creating an electric fieldduring resonance of the first resonant assembly, passing gas through theelectric field, and forming, by virtue of interaction between theelectric field and the gas, a plasma; subsequent to the formation of aplasma, applying the oscillating electrical output signal to the secondpair of electrodes and causing resonance of a second resonant assemblyof which the second pair of electrodes form a part, and maintaining theplasma by delivering into the plasma via the second pair of electrodes,power from the oscillating output signal. The oscillating output signalmay remain substantially constant. The first and second pairs ofelectrodes may be distinct, or they may have an electrode common toboth. The electric field is preferably formed between the first pair ofelectrodes, but may be formed between an electrode of the first pair ofelectrodes and an electrode of the second pair of electrodes, in whichcase the electric field may be formed between two electrodes, both ofwhich are connected to the same output terminal of the power outputdevice.

As a result the preferred method, the plasma causes necrosis of livingepidermal cells and vaporisation of dead epidermal cells, and whererequired, produces effects in the dermis.

According to another aspect of the invention, a system for controllingthe use of a device for treating human tissue comprises: a generator forproviding pulses of energy to the device, the pulses of energy being atselected energy levels; a controller for controlling the supply ofenergy to the device; an electronic key associated with the device andincluding memory means; and a read/write device associated with thecontroller for downloading information from the electronic key andwriting information to the memory means; the controller causing theread/write device to send signals to the memory means when energy pulsesare provided to the device by the generator to cause updating of adevice usage counter in the memory means, whereby the rate at which thecounter is incremented increases as the delivered power is increased,the controller further causing the generator to cease the provision ofpulses when the device usage counter reaches a predetermined maximumvalue.

According to another aspect of the invention, there is provided a systemfor controlling the use of a device for treating human tissue, thesystem comprising a generator for providing pulses of energy to thedevice, the pulses of energy being at selected energy levels, acontroller for controlling the supply of energy to the device, anelectronic key associated with each device and including memory means,and a read/write device associated with the controller for downloadinginformation from the electronic key and writing information to thememory means, the controller causing the read/write device to send asignal to the memory means to update an incremental counter each time apredetermined amount of energy is provided to the device, the controllercausing the generator to cease the provision of pulses when theincremental counter reaches a predetermined maximum value, characterisedin that the controller updates the incremental counter by a first valuefor every pulse provided to the device which is below a predeterminedthreshold energy level, and updates the incremental counter by a secondlarger value for every pulse provided to the device which is above thepredetermined threshold energy level.

According to a further aspect of the invention, a system for controllingthe use of a device for treating human tissue comprises: a generator forcontrolling the use of a device for treating human tissue comprises: agenerator for providing pulses of energy to the device, the pulses ofenergy being at selected energy levels; a controller for controlling thesupply of energy to the device; an electronic key associated with thedevice and including memory means; and a read/write device associatedwith the controller for downloading information from the electronic keyand writing information to the memory means; the controller causing theread/write device to send signals to the memory means when energy pulsesare provided to the device by the generator to cause updating of adevice usage counter in the memory means whereby the counter isincremented at different rates according to the energy of the pulses sothat pulses of a first energy level cause incrementing of the countermore quickly than pulses of a second energy level, the first energylevel being higher than the second energy level.

There are a number of electronic key systems on the market, such as the“i-button” system from Dallas Semiconductor Corp. These can be used fora variety of purposes, including personnel access and e-commerceapplications. Examples of electronic key systems proposed for use withmedical apparatus include U.S. Pat. No. 6,464,689 assigned to CuronMedical and U.S. Pat. No. 5,742,718 assigned to Eclipse SurgicalTechnologies. Prior art systems such as these can be used toauthenticate a disposable handpiece by having the control unit registera unique code carried by the electronic key. These systems can alsostore usage data for the medical instrument, and patient data for theprocedure being undertaken. The present invention provides a simplesystem, yet one that takes into account that the electronic key may bepresented to different control units in an attempt to obtain additionalusage time. The system also takes into account that the device may beused at different power settings, and the acceptable usage time maydepend upon these power settings.

The preferred system sets a threshold energy level, and increments thecounter by a different amount depending on whether the energy suppliedis above or below the threshold level. Depending on the memoryavailable, the counter can be incremented every time a pulse is providedto the device, or every time a predetermined amount of energy (i.e. apreset number of pulses) is provided to the device. Conveniently thepredetermined threshold energy level is in the range of 0.5 to 2.5Joules, and typically substantially 2 Joules.

According to a convenient arrangement, the second value (for energysupplied above the threshold) is substantially twice the value of thefirst value (for energy supplied below the threshold). In onearrangement, the predetermined maximum value is between 500 and 5000times the first value, typically between 2000 and 3000 times the firstvalue. Thus the system gives up to 5000 pulses at the lower energylevels, or up to 2500 pulses at the higher energy levels.

Alternatively, the controller configuration may be such that the counteris incremented at more than two different rates according to pulseenergy, using two or more different energy thresholds. According toanother configuration, the rate of incrementing of the counter mayincrease progressively as the delivered power increases.

Conveniently, the memory means includes a unique identifying code forthe electronic key, and /or the controller writes a unique identifyingcode to the memory means. Additionally, the controller is adapted tocause the read/write device to send a signal to the memory meansrepresentative of the time that the electronic key is first presented tothe read/write device, or the time that the first pulses of energy areprovided to the device. The controller is preferably adapted to comparethe current time with the time that the key was presented or the firstpulses of energy were provided to the device, and to prevent theprovision of pulses when the time difference exceeds a predeterminedvalue. Thus not only can the system confirm the identity of thedisposable element presented to the generator, but it can also identifydisposable elements that are old in that they were first used prior to agiven period of time. Conveniently, the predetermined value is between 6and 12 hours.

The invention also extends to a method of controlling the use of adevice for treating human tissue comprising the steps of providing acontroller for controlling the supply of energy to the device, and anelectronic key associated with each device, the electronic key includingmemory means, presenting the electronic key to the controller, readingthe value from an incremental counter on the memory means anddetermining whether it has reached a predetermined maximum value,supplying pulses of energy to the device if the incremental counter hasnot reached its predetermined maximum value, updating the incrementalcounter at a rate which increases as the power provided to the device isincreased. The increase in the rate of incrementing the counter as thepower increases may be brought about by updating the incremental counterby a first value each time that a pulse or a number of pulses isprovided to the device below a predetermined threshold energy level, andupdating the incremental counter by a second larger value each time thata pulse or a number of pulses is provided to the device above thepredetermined threshold energy level.

The invention further resides in a method of skin treatment, comprisingthe steps of generating pulses of plasma, and, in an initial treatment,applying at least one pulse of plasma at a first energy level to asurface of skin, wherein the application of said at least one pulse ofplasma to the surface of skin causes denaturation of collagen withincollagen-containing tissue beneath the skin surface without causing thecomplete removal of epidermis at said surface of skin, and, in asubsequent treatment, applying at least one pulse of plasma at a secondenergy level to a surface of skin, wherein the application of said atleast one pulse of plasma to the surface of skin causes the destructionof the majority of the epidermis.

The initial treatment may be constituted by a single session at whichone or more pulses are applied to each area of skin to be treated, orseveral of such sessions repeated at a time interval ranging fromseveral minutes to a month or more. In the initial treatment, eachsession results in energy being delivered below that resulting in thedestruction of the epidermis.

The subsequent treatment is constituted by a single session, which mayinvolve one or more pulses being applied to each area of skin to betreated. The energy delivered, whether from a single pulse or a seriesof pulses, is such that the majority of the epidermis is destroyed.

The present invention allows for the control of the energy delivered,whether in single or multiple sessions, such that the useful life of theinstrument is maximized without risking the degradation of theinstrument. Degradation of the instrument is to be avoided so as tominimize the possibility of variations in the energy being delivered tothe skin being treated, and hence the effect the treatment may have onthe tissue to be treated.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleand with reference to the accompanying drawings, in which:

FIG. 1 is a schematic drawing illustrating the principle underlying asurgical system for skin resurfacing according to the present invention;

FIG. 2 is a longitudinal cross-section of a surgical instrument for usein a system in accordance with the present invention;

FIG. 3 is a detail of FIG. 2;

FIG. 4 is a schematic illustration of a generator used in conjunctionwith the instrument of FIGS. 2 and 3;

FIG. 5 is a graph showing reflected power as a function of operatingfrequency;

FIG. 6 is a cross-section showing a modification of part of theinstrument shown in FIG. 3;

FIG. 7 is a schematic drawing of an alternative generator including amagnetron;

FIG. 8 is a more detailed block diagram of a generator including amagnetron;

FIG. 9 is a circuit diagram of an inverter unit of the generator of FIG.8;

FIG. 10 is a graph illustrating the switch-on characteristics of themagnetron in the generator of FIG. 8;

FIG. 11 is a block diagram of an outer power control loop of thegenerator of FIG. 8;

FIG. 12 is a block diagram of intermediate and inner power control loopsof the generator of FIG. 8;

FIG. 13 is a cross section of a UHF isolator forming part of thegenerator of FIG. 8;

FIG. 14 is a section through an embodiment of instrument suitable foruse with the generator of FIG. 7;

FIG. 15 is a graph of reflected power versus frequency for theinstrument of FIG. 14 when employed with the generator of FIG. 7;

FIG. 16 is a section through a further embodiment of instrument;

FIG. 17 is a graph of reflected power versus frequency in the instrumentof FIG. 16;

FIG. 18 is a schematic illustration of a further embodiment ofinstrument.

FIG. 19 is a cut-away perspective view of another alternativeinstrument; and

FIG. 20 is a longitudinal cross-section of part of the instrument ofFIG. 19.

FIG. 21 is a perspective view of an instrument for use in the surgicalsystem of FIG. 1,

FIG. 22 is a sectional side view of the instrument of FIG. 21,

FIG. 23 is a sectional side view of an electrode used in the instrumentof FIG. 21,

FIG. 24 is a sectional side view of a disposable assembly, used in theinstrument of FIG. 21, and

FIG. 25 is a schematic illustration of a system according to a furtheraspect of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, the principle of operation of embodiments of theinvention will now be described. A surgical system comprises a generator4 which includes a power output 6, typically in the form of anoscillator and an amplifier, or a thermionic power device, and a userinterface 8 and a controller 10. The generator produces an output whichis coupled via a feed structure including a cable 12 to an electrode 14of an instrument 16. The system further includes a supply 18 of gas,which is supplied to the instrument by means of a pipe 20. The gas ispreferably a gas that enables relatively high energy to be delivered tothe tissue per unit energy delivered into the gas at the instrument.Preferably the gas should include a diatomic gas (or gas having morethan two atoms), for example, nitrogen, carbon dioxide or air. In use,the generator operates to establish an electric field in the region ofthe tip 22 of the electrode. Gas from the supply 18 is passed throughthe electric field. If the field is sufficiently strong, it will havethe effect of accelerating free electrons sufficiently to causecollisions with the gas molecules, the result of which is either thedissociation of one or more electrons from the gas molecules to creategaseous ions, or the excitation of electrons in the gas molecules tohigher energy states, or dissociation of molecules into constituentatoms, or the excitation of vibrational states in the gaseous molecules.The result in macroscopic terms is the creation of a plasma 24 which ishot. Energy is released from the plasma by way of recombination ofelectrons and ions to form neutrally charged atoms or molecules and therelaxation to lower energy states from higher energy states. Such energyrelease includes the emission of electromagnetic radiation, for example,as light, with a spectrum that is characteristic of the gas used. Thetemperature of the plasma depends upon the nature of the gas and theamount of power delivered to the gas from the electric field (i.e. theamount of energy transferred to a given quantity of gas).

In the preferred embodiment, a low-temperature plasma is formed innitrogen. This is also known in the art as a Lewis-Rayleigh Afterglowand energy storage by the plasma is dominated by vibrational states ofthe gaseous molecule and elevated states of electrons still bound tomolecules (known as ‘metastable states’ because of their relatively longlifetime before decay to a lower energy states occurs).

In this condition the plasma will readily react, that is, give energy updue to collision, with other molecules. The plasma emits acharacteristic yellow/orange light with a principle wavelength of about580 nm.

The relatively long-lived states of the plasma is an advantage in thatthe plasma still contains useful amounts of energy by the time itreaches the tissue to be treated.

The resulting plasma is directed out of an open end of the instrumentand towards the tissue of a patient, to cause modification or partial ortotal removal thereof.

Upon impact, the nitrogen plasma penetrates a short distance into thetissue and rapidly decays into a low energy state to reach equilibriumwith its surroundings. Energy is transferred through collisions (thusheating the tissue) and emission of electromagnetic energy with aspectrum typically extending from 250 to 2500 nm. The electromagneticenergy is absorbed by the tissue with consequent heating.

Where the system is employed for the purpose of skin resurfacing, thereare a variety of skin resurfacing effects which may be achieved by theapplication of a plasma to the skin, and different effects are achievedby delivering different amounts of energy to the skin for differentperiods of time. The system operates by generating a plasma in shortpulses. The various combinations of these parameters result in differentskin resurfacing effects. For example, applying relatively high power inextremely short pulses (i.e. over an extremely short period of time)will result in the virtual instantaneous vaporisation of an uppermostlayer of the epidermis (i.e. dissociation into tiny fragments, which inthis situation are usually airborne). The high power delivery results inthe vaporisation of the tissue, while the short time period over whichenergy is delivered prevents deeper penetration of thermally inducedtissue damage. To deliver high power levels to the tissue, a hightemperature plasma is required, and this can be obtained by deliveringenergy at a high level into a given quantity of gas (i.e. high energyover a short period of time, or high power) from the electric field. Itshould be noted that the temperature of the plasma decreases withincreasing distance from the electrode tip, which means that thestand-off distance of the instrument from the surface of the skin willaffect the temperature of the plasma incident upon the skin and,therefore, the energy delivered to the skin over a given time period.This is a relatively superficial skin resurfacing treatment, but has theadvantage of extremely short healing times.

A deeper effect, caused by thermal modification and eventual removal ofa greater thickness of tissue, may be obtained by delivering lowerlevels of power to the skin but for longer periods of time. A lowerpower level and, thus, a lower rate of energy delivery avoidssubstantially instantaneous vaporisation of tissue, but the longerperiod over which power is delivered results in a greater net energydelivery to the tissue and deeper thermal effects in the tissue. Theresultant blistering of the skin and subsequent tissue necrosis occurover a substantially longer period of time than in the case of asuperficial treatment. The most deeply penetrative skin resurfacing,which may involve an stepwise process whereby several “passes” are madeover the tissue so that a given area of skin is exposed to the plasma ontwo or more occasions, can penetrate sufficiently deeply to cause thedenaturing of collagen in the dermis. This has applicability in theremoval or remodelling of scar tissue (such as that caused by acne, forexample), and reduction of wrinkles. Depilation of the skin surface mayalso be achieved.

The system and methods of the present invention may also be used todebride wounds or ulcers, or in the treatment of a variety of cutaneousor dermatological disorders. including: malignant tumours (whetherprimarily or secondarily involving the skin); port wine stains;telangiectasia; granulomas; adenomas; haemangioma; pigmented lesions;nevi; hyperplastic, proliferative and inflammatory fibrous papules;rhinophyma; seborrhoeic heratoses; lymphocytoma; angiofibromata; warts;neurofibromas; condylomata; keliod or hypertrophic scar tissue.

The system and methods of the present invention also have applicabilityto numerous other disorders, and in this regard the ability to vary thedepth of tissue effect in a very controlled manner is particularlyadvantageous. For example, in a superficial mode of treatment, tissuesurfaces of the body other than skin may be treated, including thelinings of the oropharynx, respiratory and gastrointestinal tracts inwhich it is desirable to remove surface lesions, such as leudoplakia (asuperficial pre-cancerous lesion often found in the oropharynx), whileminimising damage to underlying structures. In addition, the peritonealsurface of organs and structures within the abdomen may be a site forabnormal implantation of endometrial tissue derived from the uterus.These are often constituted by superficial plaques which may also betreated using the invention set in a superficial mode of treatment. Ifsuch lesions involve deeper layers of tissue then these may be treatedby multiple applications using the invention or the depth of tissueeffect may be adjusted using the control features included within theinvention and which are further described herein.

By employing a system or method in accordance with the invention with asetting designed to achieve a deeper effect, tissue structures deep tothe surface layer may be treated or modified. Such modification mayinclude the contraction of collagen containing tissue often found intissue layers deep to the surface layer. The depth control of the systemallows vital structures to be treated without, for instance, causingperforation of the structure. Such structures may include parts of theintestine where it is desirable to reduce their volume, such as ingastroplexy (reducing the volume of the stomach), or in instances wherethe intestine includes abnormal out-pouchings or diverticular. Suchstructures may also include blood vessels which have become abnormallydistended by an aneurysm or varicosisties, common sites being the aorticartery, the vessels of the brain or in the superficial veins of the leg.Apart from these vital structures, musculo-skeletal structures may alsobe modified where they have become stretched or lax. A hiatus herniaoccurs when a portion of the stomach passes through the crura of thediaphragm which could, for example, be modified using the instrumentsuch that the aperture for the stomach to pass through is narrowed to apoint at which this does not occur by contracting the crura. Hernias inother areas of the body may be similarly treated including by modifyingcollagen-containing structures surrounding the weakness through whichthe herniation occurs. Such hernias include but are not limited toinguinal and other abdominal hernias.

Various embodiments of system for tissue resurfacing will now bedescribed in further detail. Referring to FIGS. 2 and 3, a skinresurfacing instrument 16 has an outer shaft 30 with has a connector 26at its proximal end, by means of which the instrument may be connectedto the output terminals of a generator (described in more detail withreference to FIG. 4), usually via a flexible cable, as shown in FIG. 1.The instrument also receives a supply of nitrogen at inlet port 32,which is fed initially along an annular conduit 34 formed between shaft30 and a length of coaxial feed cable 40, and subsequently, viaapertures 36 along a further sections of annular conduit 38A and 38B.The sections 38A, 38B of annular conduit are formed between a conductivesleeve 50, which is connected to the outer conductor 44 of the coaxialfeed cable, and conductive elements 52 and 54 respectively which areconnected to the inner conductor 42 of the coaxial feed cable 40. At thedistal end of the annular conduit 38B the gas is converted into a plasmaunder the influence of an oscillating high intensity electric field Ebetween an inner needle-like electrode 60 provided by the distal end ofthe conductive element 54, and an outer second electrode 70 provided bya part of the sleeve 50 which is adjacent and coextensive with theneedle electrode 60. The resultant plasma 72 passes out of an aperture80 formed in a ceramic disc 82 in the distal end of the instrumentlargely under the influence of the pressure from the nitrogen supply,the insulating nature of the disc 82 serving to reduce or avoidpreferential arcing between the electrodes 60 and 70.

The inner electrode 60 is connected to one of the generator outputterminals via the conductive elements 52, 54 and the inner conductor 42of the coaxial feed structure, and the outer electrode 70 is connectedto the other generator output terminal via the conductive sleeve 50 andthe outer conductor 44 of the coaxial feed structure 40. (Waveguides mayalso be used as the feed structure.) The intensity of the electric fieldbetween them therefore, oscillates at the output frequency of thegenerator, which in this embodiment is in the region of 2450 MHz. Inorder to generate a plasma from the nitrogen gas, a high intensityelectric field is required. In this regard the relatively pointedconfiguration of the needle electrode 60 assists in the creation of sucha field, because charge accumulates in the region of the tip, which hasthe effect of increasing the field intensity in that region. However,the creation of a high intensity electric field requires a largepotential difference between the inner and outer electrodes 60, 70 and,generally speaking, the magnitude of the potential difference requiredto create such a field increases with increasing separation of theelectrodes. The electric field intensity required to strike a plasmafrom nitrogen (and thus create a plasma) is in the region of 3 M.Newtonsper Coulomb of charge, which translated into a uniform potentialdifference, equates roughly to a potential difference of 3 kV betweenconductors separated by a distance of 1 mm. In the instrumentillustrated in FIG. 2, the separation between the inner and outerelectrodes 60, 70 is approximately 3 mm, so that were the field uniformthe voltage required to achieve the requisite field intensity would beapproximately 10 kV. However the geometry of the electrode 60 is such asto concentrate charge in regions of conductor which have a smallcurvature thereby intensifying the electric field regions adjacent suchconductors and reducing the magnitude of potential difference which mustbe supplied to the electrodes in order to create a field of the requiredstrength. Nonetheless, in practice it is not necessarily desirable tosupply a potential difference of sufficient magnitude to the electrodes60, 70 directly from the generator, because the insulator of the feedstructure used to connect the generator output to the electrodes 60, 70may be subject to breakdown.

In the embodiment described above with reference to FIGS. 1 to 3, theoutput voltage of the generator is preferably of the order of 100 V. Inorder to obtain a high enough voltage across the electrodes 60, 70 tostrike a plasma, therefore, it is necessary to provide a step-up, orupward transformation of the supply voltage from the generator. One wayof achieving this is to create a resonant structure which incorporatesthe electrodes 60, 70. If an output signal from the generator issupplied to the resonant structure (and, therefore, the electrodes) at afrequency which is equal to or similar to its resonant frequency, theresulting resonance provides voltage multiplication of the generatoroutput signal across the electrodes 60, 70 the magnitude of which isdetermined by the geometry of the structure, the materials used withinthe structure (e.g. the dielectric materials), and the impedance of aload. In this instrument, the resonant structure is provided by acombination of two impedance matching structures 92, 94 the function andoperation of which will be described in more detail subsequently.

The use of a resonant structure is one way of providing a sufficientlyhigh voltage across the electrodes 60, 70 to strike a plasma. For theinstrument to be effective, however, it is necessary for the generatorto deliver a predetermined and controllable level of power to theplasma, since this affects the extent to which the nitrogen is convertedinto plasma, which in turn affects the energy which may be delivered tothe tissue in the form of heat. In addition it is desirable to haveefficient transmission of power from the generator to the load providedby the plasma. As mentioned above, the output frequency of the generatorin the present example is in the ultra high frequency (UHF) band offrequencies, and lies in the region of 2450 MHz, this being a frequencywhose use is permitted for surgical purposes by ISM legislation. Atfrequencies of this magnitude is appropriate to consider thetransmission of electrical signals in the context of such a surgicalsystem as the transmission of electromagnetic waves, and the feedstructures for their efficient propagation of taking the form of coaxialor waveguide transmission lines.

In the instrument of FIG. 2, the coaxial cable 40 provides thetransmission line feed structure from the generator 4 to the instrument16. The inner and outer conductors 42, 44 of the coaxial feed structure40 are spaced from each other by an annular dielectric 46. To provideefficient transmission of power from the output of the generator using atransmission line, the internal impedance of the generator is desirablyequal to the characteristic impedance of the transmission line. In thepresent example the internal impedance of the generator is 50 ohms, andthe characteristic impedance of the coaxial cable 40 is also 50 ohms.The load provided to the generator prior to striking plasma is of theorder of 5 k ohms. Owing to this large difference in impedance betweenthe generator impedance and feed structure on the one hand, and the loadon the other, delivering power to the load directly from the feedstructure will result in substantial losses of power (i.e. power outputfrom the generator which is not delivered to the load) due toreflections of the electromagnetic waves at the interface between thefeed structure and the load. Thus, it is not preferable simply toconnect the inner and outer conductors 42, 44 of the coaxial cable 40 tothe electrodes 60, 70 because of the resultant losses. To mitigateagainst such losses it is necessary to match the relatively lowcharacteristic impedance of the cable 40 and the relatively high loadimpedance, and in the present embodiment this is achieved by connectingthe load to the feed structure (whose characteristic impedance is equalto that of the generator impedance) via an impedance transformerprovided by two sections 92, 94 of transmission line having differentcharacteristic impedances to provide a transition between the lowcharacteristic impedance of the coaxial feed structure and the highimpedance load. The matching structure 92 has an inner conductorprovided by the conductive element 52, which has a relatively largediameter, and is spaced from an outer conductor provided by theconductive sleeve 50 by means of two dielectric spacers 56. As can beseen from FIG. 2, the spacing between the inner and outer conductors 52,50 is relatively small, as a result of which the matching structure 92has a relatively low characteristic impedance (in the region of 8 ohmsin this embodiment). The matching structure 94 has an inner conductorprovided by the conductive element 54, and an outer conductor providedby the sleeve 50. The inner conductor provided by the conductive element54 has a significantly smaller diameter than conductive element 52, andthe relatively large gap between the inner and outer conductors 50, 54results in a relatively high characteristic impedance (80 ohms) of thematching structure 94.

Electrically, and when operational, the instrument may be thought of asfour sections of different impedances connected in series: the impedanceZ_(F) of the feed structure provided by the coaxial cable 40, theimpedance of the transition structure provided by the two seriesconnected matching structures 92, 94 of transmission line, havingimpedances Z₉₂ and Z₉₄ respectively, and the impedance Z_(L) of the loadprovided by the plasma which forms in the region of the needle electrode60. Where each of the sections 92, 94 of the matching structure has anelectrical length equal to one quarter wavelength at 2450 MHz, thefollowing relationship between impedances applies when the impedance ofthe load and the feed structure are matched:Z _(L) /Z _(F) =Z ₉₄ ² /Z ₉₂ ²

The impedance Z_(L) of the load provided to the generator by the plasmais in the region of 5 k ohms; the characteristic impedance Z_(F) of thecoaxial cable 40 is 50 ohms, meaning that the ratio Z₉₄ ²/Z₉₂ ²=100 andso Z₉₄/Z₉₂=10. Practical values have been found to be 80 ohms for Z₉₄,the impedance of the matching structure section 94, and 8 ohms for Z₉₂,the impedance of matching structure section 92.

The requirement that each of the matching structures 92, 94 are onequarter wavelength long is an inherent part of the matching process. Itssignificance lies in that at each of the interfaces between differentcharacteristic impedances there will be reflections of theelectromagnetic waves. By making the sections 92, 94 one quarterwavelength long, the reflections at e.g. the interface between thecoaxial feed structure 40 and the section 92 will be in anti phase withthe reflections at the interface between the section 92 and the section94, and so will destructively interfere; the same applies to thereflections at the interfaces between the sections 92 and 94 on the onehand and the reflections at the interface between section 94 and theload on the other. The destructive interference has the effect ofminimising power losses due to reflected waves at interfaces betweendiffering impedances, provided that the net reflections of theelectromagnetic waves having nominal phase angle of 0 radians are ofequal intensity to the net reflections having a nominal phase angle of πradians (a condition which is satisfied by selecting appropriateimpedance values for the different sections 92, 94).

Referring now to FIG. 4, an embodiment of generator used in conjunctionwith the embodiment of instrument described above comprises a powersupply unit 100, which receives an alternating current mains input andproduces a constant DC voltage across a pair of output terminals 102,which are connected to a fixed gain solid state power amplifier 104. Thepower amplifier 104 receives an input signal from a tunable oscillator106 via a variable attenuator 108. The power amplifier 104, tunableoscillator 106, and variable attenuator 108 may be thought of as an ACpower output device. Control of the frequency of oscillation of theoscillator, and the attenuator 108 is performed by means of voltageoutputs V_(tune) and V_(gain) from a controller 110 (the operation ofwhich will subsequently be described in more detail) in dependence uponfeedback signals, and input signals from a user interface 112. Theoutput of the amplifier 104 passes through a circulator 114, and thensequentially through output and return directional couplers 116,118which in conjunction with detectors 120,122 provide an indication of thepower output P_(out) by the generator and the power reflected Pref backinto the generator respectively. Power reflected back into the generatorpasses through the circulator 114 which directs the reflected power intoan attenuating resistor 124, whose impedance is chosen so that itprovides a good match with the feed structure 40 (i.e. 50 ohms). Theattenuating resistor has the function of dissipating the reflectedpower, and does this by converting the reflected power into heat.

The controller 110 receives input signals I_(user), P_(out), P_(Ref),G_(flow) from the user interface, the output and reflected powerdetectors 120,122 and a gas flow regulator 130, respectively, the lattercontrolling the rate of delivery of nitrogen. Each of the input signalspasses through an analogue to digital converter 132 and into amicroprocessor 134. The microprocessor 134 operates, via a digital toanalogue converter 136 to control the value of three output controlparameters: V_(tune) which controls the tuning output frequency of theoscillator 106; V_(gain) which controls the extent of attenuation withinthe variable attenuator 108 and therefore effectively the gain of theamplifier 104; and G_(flow) the rate of flow of gas through theinstrument, with the aim of optimising the performance of the system.This optimisation includes tuning the output of the oscillator 106 tothe most efficient frequency of operation, i.e. the frequency at whichmost power is transferred into the plasma. The oscillator 106 maygenerate output signals throughout the ISM bandwidth of 2400-2500 MHz.To achieve optimisation of the operating frequency, upon switch-on ofthe system, the microprocessor 134 adjusts the V_(gain), output to causethe attenuator to reduce the generator output power to an extremely lowlevel, and sweeps the frequency adjusting voltage output V_(tune) fromits lowest to its highest level, causing the oscillator to sweepcorrespondingly through its 100 MHz output bandwidth. Values ofreflected power P_(ref) are recorded by the microprocessor 134throughout the bandwidth of the oscillator, and FIG. 5 illustrates atypical relationship between output frequency of the generator andreflected power P_(ref). It can be seen from FIG. 5 that the lowestlevel of reflected power occurs at a frequency f_(ref), whichcorresponds to the resonant frequency of the resonant structure withinthe instrument 16. Having determined from an initial low power frequencysweep the value of the most efficient frequency at which power may bedelivered to the electrode, the microprocessor then tunes the oscillatoroutput frequency to the frequency fres. In a modification, thecontroller is operable via a demand signal from the user interface (thedemand signal being by a user via the user interface) to perform aninitial frequency sweep prior to connection of the instrument 16 to thegenerator. This enables the controller to map the feed structure betweenthe power output device and the instrument to take account of the effectof any mismatches between discrete sections of the feed structure etc.,which have an effect upon the attenuation of power at variousfrequencies. This frequency mapping may then be used by the controller110 to ensure that it takes account only of variations in theattenuation of power with frequency which are not endemically present asa result of components of the generator and/or feed structure betweenthe generator and the instrument.

The operational power output of the power output device is set inaccordance with the input signal I_(user) to the controller 110 from theuser interface 112, and which represents a level of demanded power setin the user interface 112 by an operator. The various possible controlmodes of the generator depend upon the user interface 112, and moreparticularly the options which the user interface is programmed to giveto a user. For example, as mentioned above, there are a number ofparameters which may be adjusted to achieve different tissue effects,such as power level, gas flow rate, the length of the time period (thetreatment pulse width) for which the instrument is operational togenerate plasma over a particular region of the skin, and the stand-offdistance between the aperture at the distal end of the instrument 16 andthe tissue. The user interface 112 offers the user a number ofalternative control modes each of which will allow the user to controlthe system in accordance with differing demand criteria For example, apreferred mode of operation is one which mimics the operational controlof laser resurfacing apparatus, since this has the advantage of beingreadily understood by those currently practising in the field of skinresurfacing. In the laser resurfacing mode of operation, the userinterface invites a user to select a level of energy delivery persurface area (known in the art as “fluence”) per pulse of theinstrument. When operating in this mode, the microprocessor setsV_(gain) so that the power output device has a pre-set constant outputpower, typically in the region of 160 W, and the input signal I_(user)from the user is converted into a demanded time period represented bythe pulse width, calculated from the required energy per treatment pulseand the constant level of output power. However, the voltage signalV_(gain) is also used to switch the generator output on and off inaccordance with input signals I_(user) from the user interface. Thus,for example, when the user presses a button on the handle of theinstrument (not shown), a signal sent by the user interface 112 to themicroprocessor 134, which then operates to produce a pulse ofpredetermined width (e.g. 20 ms) by altering V_(gain) from its quiescentsetting, at which the attenuator output 108 is such that there isvirtually no signal for the amplifier 104 to amplify, and the generatoroutput is negligible, to a value corresponding to the pre-set constantoutput power for a period of time equal to the demanded pulse width.This will have the effect of altering the amplifier output from itsquiescent level to the pre-set constant output power level for a timeperiod equal to the demanded pulse width, and ultimately of creating aplasma for such a time period. By altering the pulse width according touser input, pulses of selected energies can be delivered, typically, inthe range of from 6 ms to 20 ms. These pulses can be delivered on a“one-shot” basis or as a continuous train of pulses at a predeterminedpulse frequency.

The surface area over which the energy is delivered will typically be afunction of the geometry of the instrument, and this may be entered intothe user interface in a number of ways. In one embodiment the userinterface stores surface area data for each different geometry ofinstrument that may be used with the generator, and the instrument inoperation is either identified manually by the user in response to aprompt by the user interface 112, or is identified automatically byvirtue of an identification artefact on the instrument which isdetectable by the controller (which may require a connection between thecontroller and the instrument). Additionally the surface area will alsobe a function of the stand-off distance of the instrument aperture 82from the tissue, since the greater the stand-off the cooler the plasmawill be by the time it reaches the surface, and also, depending on theinstrument geometry, the instrument may produce a divergent beam.Instruments may be operated with a fixed stand-off distance, for exampleby virtue of a spacer connected to the distal end of the instrument, inwhich case the surface area data held within the user interface willautomatically take account of the stand-off distance. Alternatively theinstruments may be operated with a variable stand-off distance, in whichcase the stand-off distance must be measured, and fed back to thecontroller to enable it to be taken into account in the surface areacalculation.

A further parameter which can affect the energy per unit area is the gasflow rate, and in one preferred embodiment the controller preferablycontains a look-up table 140 of flow rate G_(flow) against generatoroutput power P_(out) for a variety of constant output power levels, andthe flow rate for a given output power level is adjusted accordingly. Ina further modification the gas flow rate may be adjusted dynamically totake account of variations in stand-off distance, for example, and ispreferably switched off between pulses.

As described above, for optimum ease of use in the resurfacing mode, thepower output device will ideally deliver a constant output power overthe entire duration of an output, since this facilitates easy control ofthe total energy output in a given pulse. With a constant power output,the controller is able to control the total energy delivered per pulsesimply switching the power output device on (by the means of the signalV_(gain)) for a predetermined period of time, calculated on the basis ofthe output power level. It may, however, in practice be the case thatthe power output varies to a significant extent with regard to theaccuracy to within which it is required to determine to the total energydelivered per output pulse. In this case the microprocessor isprogrammed to monitor the output power by integrating P_(out) (fromdetector 120) with respect to time, and switching the power outputdevice off by altering V_(gain) to return the variable attenuator 108 toits quiescent setting.

A further complication in the control of the operation of the systemarises in that the creation of a plasma at aperture 80 amounts insimplistic electrical terms to extending the length of the needleelectrode 60, since the plasma is made up of ionised molecules, and istherefore conductive. This has the effect of lowering the resonantfrequency of the resonant structure, so that the optimum generatoroutput at which power may be delivered to the instrument for the purposeof striking a plasma is different to the optimum frequency at whichpower may be delivered into an existent plasma. To deal with thisdifficulty, the microprocessor 134 is programmed continuously to tunethe oscillator output during operation of the system. In one preferredmode the technique of “dither” is employed, whereby the microprocessor134 causes the oscillator output momentarily to generate outputs atfrequencies 4 MHz below and above the current output frequency, and thensamples, via the reflected power detector 122 the attenuation of powerat those frequencies. In the event that more power is attenuated at oneof those frequencies than at the current frequency of operation, themicroprocessor re-tunes the oscillator output to that frequency at whichgreater power attenuation occurred, and then repeats the process. In afurther preferred mode of operation, the microprocessor 134 records themagnitude of the shift in resonant frequency when a plasma is struck,and in subsequent pulses, shifts the frequency of the oscillator 106correspondingly when the system goes out of tune (i.e. when a plasma isstruck), whereupon the technique of dither is then employed. This hasthe advantage of providing a more rapid re-tuning of the system once aplasma is first struck.

As mentioned above, in the embodiment shown in FIG. 4, the amplifier 104is typically set to produce around 160 W of output power. However, notall of this is delivered into the plasma Typically power is also lostthrough radiation from the end of the instrument in the form ofelectromagnetic waves, from reflection at connections between cables,and in the form of dielectric and conductive losses (i.e. theattenuation of power within the dielectrics which form part of thetransmission line). In the instrument design of FIGS. 2 and 3 it ispossible to take advantage of dielectric loss by virtue of feeding thegas through the annular conduits 38A,B of the sections 92, 94 of theimpedance matching structure; in this way, dielectric power losses intothe gas serve to heat up the gas, making it more susceptible toconversion into a plasma

Referring now to FIG. 6, in a modification of the instrument 14 shown inFIGS. 2 and 3, an end cap 84, made of conducting material, is added tothe distal end of the instrument 14. The end cap is electricallyconnected to the sleeve 50 and is, therefore, part of the electrode 70.The provision of the end cap 84 has several beneficial effects. Firstly,since the electric field preferentially extends from conductor toconductor, and the end cap 84 effectively brings the electrode 70 closerto the tip of the needle electrode 60, it is believed that its geometryserves to increase the intensity of the electric field in the regionthrough which the plasma passes as it is expelled from the instrument,thereby accelerating ions within the plasma. Secondly, the physicaleffect of the end cap 84 on the plasma is that of directing the plasmain a more controlled manner. Thirdly the outer sheath currents on theinstrument (i.e. the current travelling up the outside of the instrumentback towards the generator) are reduced significantly with the end cap84, since the electrode 60, even when electrically extended by a plasma,extends to a lesser extent beyond the end of the instrument, and solosses of this nature are reduced.

In an alternative, and simpler embodiment of system operating at anoutput frequency in the range of 2450 MHz, a power output device capableof delivering significantly more power than a solid state amplifier maybe employed. With increased available power from the power outputdevice, the required voltage step-up is lower and so the role played byresonant structures (for example) decreases.

Accordingly, and referring now to FIG. 7, an alternative generator has ahigh voltage rectified AC supply 200 connected to a thermionic radiofrequency power device, in this case to a magnetron 204. The magnetron204 contains a filament heater (not shown) attached to the magnetroncathode 204C which acts to release electrons from the cathode 204C, andwhich is controlled by a filament power supply 206; the greater thepower supplied to the filament heater, the hotter the cathode 204Cbecomes and therefore the greater the number of electrons supplied tothe interior of the magnetron. The magnetron may have a permanent magnetto create a magnetic field in the cavity surrounding the cathode, but inthis embodiment it has an electromagnet with a number of coils (notshown) which are supplied with current from an electromagnet powersupply 208. The magnetron anode 204A has a series of resonant chambers210 arranged in a circular array around the cathode 204C and itsassociated annular cavity. Free electrons from the cathode 204C areaccelerated radially toward the anode 204A under the influence of theelectric field created at the cathode 204C by the high voltage supply200. The magnetic field from the electromagnet (not shown) acceleratesthe electrons in a direction perpendicular to that of the electricfield, as a result of which the electrons execute a curved path from thecathode 204C towards the anode 204A where they give up their energy toone of the resonant chambers 210. Power is taken from the resonantchambers 210 by a suitable coupling structure to the output terminal Theoperation of magnetron power output devices is well understood per seand will not be described further herein. As with the generator of FIG.4, a circulator (not shown in FIG. 7) and directional couplers may beprovided.

The magnetron-type power output device is capable of generatingsubstantially more power than the solid state power output device ofFIG. 4, but is more difficult to control. In general terms, the outputpower of the magnetron increases: (a) as the number of electrons passingfrom the cathode to the anode increases; (b) with increased supplyvoltage to the cathode (within a relatively narrow voltage band); (c)and with increased magnetic field within the magnetron. The high voltagesupply 200, the filament supply 206 and the electromagnetic supply 208are, therefore, all controlled from the controller in accordance withinput settings from the user interface, as in the case of the solidstate amplifier power output device. Since the magnetron is moredifficult to control, it is less straightforward to obtain a uniformpower output over the entire duration of a treatment pulse (pulse ofoutput power). In one method of control, therefore, the controlleroperates by integrating the output power with respect to time andturning the high voltage supply 200 off (thus shutting the magnetronoff) when the required level of energy has been delivered, as describedabove. Alternatively, the output of the cathode supply may be monitoredand controlled to provide control of output power by controlling thecurrent supplied, the cathode/anode current being proportional to outputpower.

A further alternative generator for use in a system in accordance withthe invention, and employing a magnetron as the power output device,will now be described with reference to FIG. 8. As in the embodiment ofFIG. 7, power for the magnetron 204 is supplied in two ways, firstly asa high DC voltage 200P for the cathode and as a filament supply 206P forthe cathode heater. These power inputs are both derived, in thisembodiment, from a power supply unit 210 having a mains voltage input211. A first output from the unit 210 is an intermediate level DC output210P in the region of 200 to 400V DC (specifically 350V DC in this case)which is fed to a DC converter in the form of a inverter unit 200 whichmultiplies the intermediate voltage to a level in excess of 2 kV DC, inthis case in the region of 4 kV.

The filament supply 206 is also powered from the power supply unit 210.Both the high voltage supply represented by the inverter unit 200 andthe filament supply 206 are coupled to a CPU controller 110 forcontrolling the power output of the magnetron 204 in a manner which willbe described hereinafter.

A user interface 112 is coupled to the controller 110 for the purpose ofsetting the power output mode, amongst other functions.

The magnetron 204 operates in the UHF band, typically at 2.475 GHz,producing an output on output line 204L which feeds a feed transitionstage 213 converting the waveguide magnetron output to a coaxial 50 ohmsfeeder, low frequency AC isolation also being provided by this stage.Thereafter, circulator 114 provides a constant 50 ohms load impedancefor the output of the feed transition stage 213. Apart from a first portcoupled to the transition stage 213, the circulator 114 has a secondport 114A coupled to a UHF isolation stage 214 and hence to the outputterminal 216 of the generator. A third port 114B of the circulator 114passes power reflected back from the generator output 216 via port 114Ato a resistive reflected power dump 124. Forward and reflected powersensing connections 116 and 118 are, in this embodiment, associated withthe first and third circulator ports 114A and 114B respectively, toprovide sensing signals for the controller 110.

The controller 110 also applies via line 218 a control signal foropening and closing a gas supply valve 220 so that nitrogen gas issupplied from source 130 to a gas supply outlet 222. A surgicalinstrument (not shown in FIG. 8) connected to the generator has alow-loss coaxial feeder cable for connection to UHF output 216 and asupply pipe for connection to the gas supply outlet 222.

It is important that the effect produced on tissue is both controllableand consistent, which means that the energy delivered to the skin shouldbe controllable and consistent during treatment. For treatment of skinor other surface tissue it is possible for apparatus in accordance withthe invention to allow a controlled amount of energy to be delivered toa small region at a time, typically a circular region with a diameter ofabout 6 mm. As mentioned above, to avoid unwanted thermal affects to adepth greater than required, it is preferred that relatively highpowered plasma delivery is used, but pulsed for rapid treatment to alimited depth. Once a small region is treated, typically with a singleburst of radio frequency energy less than 100 ms in duration (a single“treatment pulse”), the user can move the instrument to the nexttreatment region before applying energy again. Alternatively, pluralpulses can be delivered at a predetermined rate. Predictability andconsistency of affect can be achieved if the energy delivered to thetissue per pulse is controlled and consistent for a given controlsetting at the user interface. For this reason, the preferred generatorproduces a known power output and switches the radio frequency power onand off accurately. Generally, the treatment pulses are much shorterthan 100 ms, e.g. less than 30 ms duration, and can be as short as 2ms.When repeated, the repetition rate is typically in the range of from 0.5or 1 to 10 or 15 Hz.

The prime application for magnetron devices is for dielectric heating.Power control occurs by averaging over time and, commonly, the device isoperated in a discontinuous mode at mains frequency (50 or 60 Hz). Amains drive switching circuit is applied to the primary winding of thestep-up transformer, the secondary winding of which is applied to themagnetron anode and cathode terminals. Commonly, in addition, thefilament power supply is taken from an auxiliary secondary winding ofthe step-up transformer. This brings the penalty that the transientresponses of the heater and anode-cathode loads are different; theheater may have a warm-up time of ten to thirty seconds whereas theanode-cathode response is less than 10 μs, bringing unpredictable poweroutput levels after a significant break. Due to the discontinuous powerfeed at mains frequency, the peak power delivery may be three to sixtimes the average power delivery, depending on the current smoothingelements in the power supply. It will be appreciated from the pointsmade above that such operation of a magnetron is inappropriate fortissue resurfacing. The power supply unit of the preferred generator inaccordance with the present invention provides a continuous power feedfor the radio frequency power device (i.e. the magnetron in this case)which is interrupted only by the applications of the treatment pulses.In practice, the treatment pulses are injected into a power supply stagewhich has a continuous DC supply of, e.g., at least 200V. The UHFcirculator coupled to the magnetron output adds to stability byproviding a constant impedance load.

In the generator illustrated in FIG. 8, the desired controllability andconsistency of effect is achieved, firstly, by use of an independentfilament supply. The controller 110 is operated to energise themagnetron heater which is then allowed to reach a steady state beforeactuation of the high voltage supply to the magnetron cathode.

Secondly, the high voltage power supply chain avoids reliance on heavyfiltering and forms part of a magnetron current control loop having amuch faster response than control circuits using large shunt filtercapacitances. In particular, the power supply chain includes, asexplained above with reference to FIG. 8, an inverter unit providing acontinuous controllable current source applied at high voltage to themagnetron anode and cathode terminals. For maximum efficiency, thecurrent source is provided by a switched mode power supply operating ina continuous current mode. A series current-smoothing inductance in theinverter supply is fed from a buck regulator device. Referring to FIG.9, which is a simplified circuit diagram, the buck regulator comprises aMOSFET 230, the current-smoothing inductor 232 (here in the region of500 μH), and a diode 234. The buck regulator, as shown, is connectedbetween the 350V DC rail of the PSU output 210P (see FIG. 8) and abridge arrangement of four switching MOSFETs 236 to 239, forming aninverter stage. These transistors 236 to 239 are connected in anH-bridge and are operated in anti phase with slightly greater than 50%ON times to ensure a continuous supply current to the primary winding240P of the step-up transformer 240. A bridge rectifier 242 coupledacross the secondary winding 240F and a relatively small smoothingcapacitor 244, having a value less than or equal to 220 μS yields therequired high voltage supply 200P for the magnetron.

By pulsing the buck transistor 230 as a switching device at a frequencysignificantly greater than the repetition frequency of the treatmentpulses, which is typically between 1 and 10 Hz or 15 Hz, and owing tothe effect of the inductor 232, continuous current delivery at a powerlevel in excess of 1 kW can be provided for the magnetron within eachtreatment pulse. The current level is controlled by adjusting themark-to-space ratio of the drive pulses applied to the gate of the bucktransistor 230. The same gate terminal is used, in this case, incombination with a shut-down of the drive pulses to the inverter stagetransistors, to de-activate the magnetron between treatment pulses.

It will be appreciated by the skilled man in the art that singlecomponents referred to in this description, e.g. single transistors,inductors and capacitors, may be replaced by multiple such components,according to power handling requirements, and so on. Other equivalentstructures can also be used.

The pulse frequency of the buck transistor drive pulses is preferablygreater than 16 kHz for inaudability (as well as for control loopresponse and minimum current ripple) and is preferably between 40 kHzand 150 kHz. Advantageously, the inverter transistors 236 to 239 arepulsed within the same frequency ranges, preferably at half thefrequency of the buck transistor consistency between successive halfcycles applied to the step-up transformer 240.

Transformer 240 is preferably ferrite cored, and has a turns ratio of2:15.

As will be seen from FIG. 10, which shows the output voltage on output200P and the power output of the magnetron at the commencement of atreatment pulse, start-up can be achieved in a relatively short time,typically less than 300 μs, depending on the vale of the capacitor 244.Switch-off time is generally considerably shorter. This yields theadvantage that the treatment pulse length and, as a result, the energydelivered per treatment pulse (typically 2 to 6 joules) is virtuallyunaffected by limitations in the power supply or the magnetron. Highefficiency (typically 80%) can be achieved for the conversion from asupply voltage of hundreds of volts (on supply rails 228 and 229) to thehigh voltage output 200P (see FIG. 9).

Consistent control of the magnetron power output level, with rapidresponse to changing load conditions, can now be achieved using feedbackcontrol of the mark-to-space ration of the drive pulses to the bucktransistor 230. Since the power output from the magnetron is principallydependent on the anode to cathode current, the power supply controlservos are current-based. These include a control loop generating anerror voltage from a gain-multiplied difference between measured anodeto cathode current and a preset output-power-dependent current demand.The voltage error is compensated for the storage inductor current andthe gain multiplied difference determines the mark-to-space ratio of thedriving pulses supplied to the buck transistor 230, as shown in thecontrol loop diagrams of FIGS. 11 and 12.

A current-based servo action is also preferred to allow compensation formagnetron ageing resulting in increasing anode-to-cathode impedance.Accordingly, the required power delivery levels are maintained up tomagnetron failure.

Referring to FIGS. 8 and 11, variations in magnetron output power withrespect to anode/cathode current, e.g. due to magnetron ageing, arecompensated in the controller 110 for by comparing a forward powersample 250 (obtained on line 116 in FIG. 8) with a power referencesignal 252 in comparator 254. The comparator output is used as areference signal 256 for setting the magnetron anode current, thisreference signal 256 being applied to elements of the controller 110setting the duty cycle of the drive pulses to the buck transistor 230(FIG. 9), represented generally as the “magnetron principal powersupply” block 258 in FIG. 11.

Referring to FIG. 12, that principal power supply block 258 has outerand inner control loops 260 and 262. The anode current reference signal256 is compared in comparator 264 with an actual measurement 266 of thecurrent delivered to the magnetron anode to produce an error voltageV_(error). This error voltage is passed through a gain stage 268 in thecontroller 110 and yields a pulse width modulation (PWM) referencesignal at an input 270 to a further comparator 272, where it is comparedwith a representation 274 of the actual current in the primary windingof the step-up transformer (see FIG. 9). This produces a modified (PWM)control signal on line 276 which is fed to the gate of the bucktransistor 230 seen in FIG. 9, thereby regulating the transformerprimary current through operation of the buck stage 278.

The inner loop 262 has a very rapid response, and controls thetransformer primary current within each cycle of the 40 kHz drive pulsewaveform fed to the gate terminal 276 of the buck transistor 230. Theouter loop 260 operates with a longer time constant during eachtreatment pulse to control the level of the magnetron anode/cathodecurrent. It will be seen that the combined effect of the three controlloops appearing in FIGS. 11 and 12 is consistent and accurate control ofanode current and output power over a full range of time periods, i.e.short term and long term output power regulation is achieved.

The actual power setting applied to the UHF demand input 252 of theoutermost control loop, as shown in FIG. 11, depends on user selectionfor the required severity of treatment. Depth of effect can becontrolled by adjusting the duration of the treatment pulses, 6 to 20 msbeing a typical range.

The control connection between the controller 110 and the high voltagepower supply appears in FIG. 8 as a control and feedback channel 280.

It is also possible to control heater of current by a demand/feedbackline 282, e.g. to obtain the preferred steady state heater temperature.

In the case of the magnetron having an electromagnet, variation of themagnetic field strength applied to the magnetron cavity provides anothercontrol variable (as shown in FIG. 8), e.g. should lower continuouspower levels be requires.

Return loss monitored by line 116 in FIG. 8 is a measure of how muchenergy the load reflects back to the generator. At perfect match of thegenerator to the load, the return loss is infinite, while an opencircuit or short circuit load produces a zero return loss. Thecontroller may therefore employ a return loss sensing output on line 116as a means of determining load match, and in particular as a means ofidentifying an instrument or cable fault. Detection of such a fault maybe used to shut down the output power device, in the case of themagnetron 204.

The UHF isolation stage 214 shown in FIG. 8 is illustrated in moredetail in FIG. 13. As a particular aspect of the invention, thisisolation stage, which is applicable generally to electrosurgical (i.e.including tissue resurfacing) devices operating at frequencies in theUHF range and upwards, has a waveguide section 286 and, within thewaveguide section, spaced-apart ohmically separate launcher andcollector probes 288, 290 for connection to the radio frequency powerdevice (in this case the magnetron) and an output, specifically theoutput connector 216 shown in FIG. 8 in the present case. In the presentexample, the waveguide section is cylindrical and has end caps 292 oneach end. DC isolation is provided by forming the waveguide section 286in two interfitting portions 286A, 286B, one portion fitting within andbeing overlapped by the other portion with an insulating dielectriclayer 294 between the two portions in the region of the overlap.Suitable connectors, here coaxial connectors 296 are mounted in the wallof the waveguide section for feeding radio frequency energy to and fromthe probe 288, 290.

As an alternative, the waveguide may be rectangular in cross section ormay have another regular cross section.

Each probe 288, 290 is an E-field probe positioned inside the waveguidecavity as an extension of its respective coaxial connector innerconductor, the outer conductor being electrically continuous with thewaveguide wall. In the present embodiment, operable in the region of2.45 GHz, the diameter of the waveguide section is in the region of 70to 100 mm, specifically 86 mm in the present case. These and otherdimensions may be scaled according to the operating frequency.

The length of the interior cavity of the waveguide section between theprobe 288, 290 is preferably a multiple of λ_(g)/2 where λ_(g) is theguide wavelength within the cavity. The distance between each probe andits nearest end cap is in the region of an odd multiple of λ_(g)/4 (inthe present case 32 mm), and the axial extent of the overlap between thetwo waveguide portions 286A, 286B should be at least λ_(g)/4. A typicallow loss, high voltage breakdown material for the dielectric layer 294is polyimide tape.

It will be appreciated that the isolation stage provides a degree ofbandpass filtering in that the diameter of the waveguide section imposesa lower frequency limit below which standing waves cannot be supported,while high-pass filtering is provided by increasing losses withfrequency. Additional bandpass filtering characteristics are provided bythe relative spacings of the probe and the end caps. Note that thepreferred length of the waveguide section between the end caps 292 isabout λ_(g). Additional filter structures may be introduced into thewaveguide section to provide preferential attenuation of unwantedsignals.

The isolation stage forms an isolation barrier at DC and at ACfrequencies much lower than the operating frequency of the generator andcan, typically, withstand a voltage of 5 kV DC applied between the twowaveguide portions 286A, 286B.

At low frequencies, the isolation stage represents a series capacitorwith a value less than 1 μF, preventing thermionic current or singlefault currents which may cause unwanted nerve stimulation. Lower valuesof capacitance can obtained by reducing the degree of overlap betweenthe waveguide section portions 286A, 286B, or by increasing theclearance between them where they overlap.

Significant reductions in size of the isolation stage can be achieved byfilling the interior cavity with a dielectric material having a relativedielectric constant greater than unity.

As an alternative to the E-field probes 288, 290 illustrated in FIG. 13,waves may be launched and collected using H-field elements in the formof loops oriented to excite a magnetic field.

Referring now to FIG. 14, an instrument for use with a generator havinga magnetron power output device comprises, as with the instrument ofFIGS. 2, 3 and 6, an outer shaft 30, connector 26, coaxial feed cable40. A transitional impedance matching structure includes a low impedancesection 92 and a high impedance section 94, and provides a match betweenthe power output device of the generator and the load provided by theplasma, which is created in an electric field between a central discelectrode 160 and an outer electrode 70 provided by a section of theconductive sleeve adjacent the disc electrode 160. Gas passes from theinlet port 32 and along the annular conduits 38A, B formed between theinner and outer conductors of the sections 92, 94 of matching structurethrough the electric field between the electrodes 160, 70 and isconverted into a plasma under the influence of the electric field. Atubular quartz insert 180 is situated against the inside of the sleeve50, and therefore between the electrodes 160, 70. Quartz is a low lossdielectric material, and the insert has the effect of intensifying theelectric field between the electrodes, effectively bringing them closertogether, while simultaneously preventing preferential arcing betweenthem, thereby producing a more uniform beam of plasma In thisembodiment, the inner electrode 160 is a disc, and is mounted directlyonto the inner conductor 54 of the high impedance matching section, thelatter having a length which in electrical terms is one quarter of awavelength of the generator output. The disc electrode 160, because ofits relatively small length, is, when considered in combination with theelectrode 70 effectively a discrete or “lumped” capacitor, which, inconjunction with the inherent distributed inductance of the innerconductor 54 forms a series resonant electrical assembly. The shape ofthe disc electrode 160 also serves to spread the plasma output beam,thereby increasing the “footprint” of the beam on tissue, this can bedesirable in skin resurfacing since it means that a given area of tissuecan be treated with fewer “hits” from the instrument. The voltagestep-up which occurs in this resonant structure is lower in theinstrument of this embodiment than with the instrument of FIGS. 2, 3 and6, and so the step-up of the generator output voltage at the electrodes160, 70 as a result of resonance within the resonant assembly iscorrespondingly lower. One reason for this is that a magnetron poweroutput device produces a significantly higher level of power and at ahigher voltage (typically 300 Vrms), and therefore it is not necessaryto provide such a high step-up transformation, hence the lower Q of theresonant assembly.

Tuning of the output frequency of the magnetron power output device isdifficult. Nonetheless, the resonant frequency of the instrumentundergoes a shift once a plasma has been struck as a result of alowering of the load impedance (due to the higher conductivity of plasmathan air), so the problem of optimum power delivery for plasma ignitionon the one hand and plasma maintenance on the other still applies.Referring to FIG. 15, the reflected power dissipated within theinstrument prior to plasma ignition with varying frequency isillustrated by the line 300. It can be seen that the resonance withinthe instrument occurs at a frequency f_(res), represented graphically bya sharp peak, representative of a relatively high quality factor Q forthe voltage multiplication, or upward transformation that occurs withinthe instrument at resonance. The reflected power versus frequencycharacteristic curve for the instrument once a plasma has been struck isillustrated by the line 310, and it can be seen that the resonantfrequency of the instrument once a plasma has been created f_(pls) islower than that prior to ignition, and that the characteristic curve hasa much flatter peak, representative of lower quality factor Q. Since themagnetron power output device is relatively powerful, one preferred modeof operation involves selecting a resonant frequency of the instrumentsuch that the output frequency of the magnetron power output device isoperable both to benefit from resonance within the instrument to strikea plasma, and also to maintain a plasma.

Referring again to FIG. 15 the magnetron power output device has anoutput frequency f_(out) which lies between the resonant frequenciesf_(res) and f_(pls). The frequency f_(out) is shifted from the resonantfrequency f_(res) as far as possible in the direction of the resonantfrequency f_(pls) in an attempt to optimise the delivery of power intothe plasma, while still ensuring that sufficient resonance occurs withinthe instrument at f_(out) to strike a plasma. This compromise in theoutput frequency of the magnetron power output device is possible as aresult of the relatively large power output available, meaning thatsignificantly less resonance is required within the instrument, eitherin order to strike a plasma or subsequently to maintain a plasma, thanwould be the case with lower power output devices.

In a further embodiment, the instrument is constructed so that itincorporates two resonant assemblies: one which is resonant prior to theignition of a plasma and the other which is resonant subsequent toignition, wherein both of the resonant assemblies have similar orsubstantially the same resonant frequency. With an instrument of thistype it is then possible to optimise power delivery for ignition andmaintenance of a plasma at a single frequency. Referring now to FIG. 16,an instrument 16 has a connector 26 at its distal end, a coaxial feedstructure 40 extending from the connector 26 to a bipolar electrodestructure comprising a rod-like inner electrode 260 and an outerelectrode 70 provided by a section of outer conductive sleeve 50 lyingadjacent the rod electrode 260, A conductive end cap 84 defines anaperture 80 through which plasma passes, and helps to intensify theelectric field through which the plasma passes, thereby enhancing theease of power delivery into the plasma. The characteristic impedance ofthe section of transmission line formed by the electrode structure 260,70 is chosen to provide matching between the power output device and theload provided by the plasma. As will be explained subsequently, it isbelieved that the plasma load in this embodiment has a lower impedancethan in previous embodiments, which therefore makes matching easier. Inaddition the instrument comprises an auxiliary or strike electrode 260S.The strike electrode 260S comprises two elements: a predominantlyinductive element, provided in this example by a length of wire 272connected at its proximal end to the proximal end of rod electrode, anda predominantly capacitive element in series with the inductive element,which is provided in this example by a ring 274 of conductive materialconnected to the distal end of the wire 272, and which extendssubstantially coaxially with the rod electrode 260, but is spacedtherefrom.

Referring now to FIG. 17, the structure of the strike electrode 260S issuch that the inductance in the form of the wire 272 and the capacitancein the form of the ring 274 forms a resonant assembly which is resonantat the output frequency of the generator f_(out), and the characteristicvariation of reflected power with input frequency for the strikeelectrode 260S is illustrated by the line 320. By contrast, thetransmission line formed by the electrode structure 260, 70 (whosecharacteristic variation of reflected power with input frequency isillustrated by the line 330), has, prior to the ignition of a plasma, aresonant frequency f_(res) that is significantly higher than thegenerator output frequency to an extent that little or no resonance willoccur at that frequency. However, the electrode structure 260, 70 isconfigured such that, once a plasma has been formed (which can bethought of as a length of conductor extending from the rod electrode 260out of the aperture 80), it is a resonant structure at the outputfrequency of the generator, albeit a resonance at a lower Q. Thus, priorto the formation of a plasma, the strike electrode 260S is a resonantassembly which provides voltage multiplication (also known as step-uptransformation) of the generator output signal, whereas subsequent tothe formation of a plasma the electrode structure 260, 70 is a resonantassembly which will provide voltage multiplication. The electrodestructure 260S, 70 may be thought of as having a length, in electricalterms, and once a plasma has been created (and therefore including theextra length of conductor provided by the plasma) which is equal to aquarter wavelength, and so provides a good match of the generatoroutput.

When the generator output signal passes out of the coaxial feedstructure 40 the signal initially excites the strike electrode 260S intoresonance since this is resonant at the output frequency of thegenerator, but does not excite the electrode structure 260, 70, sincethis is not resonant at the output frequency of the generator until aplasma has been created. The effect of a resonance (and thereforevoltage multiplication) in the strike electrode 260S which does notoccur in the electrode structure 260, 70 is that there is a potentialdifference between the strike electrode 260S and the rod electrode 260.If this potential difference is sufficiently large to create an electricfield of the required intensity between the strike electrode 260S andthe rod electrode 260 (bearing in mind that, because of the relativelysmall distance between the electrodes 260S and 260, a relatively lowpotential difference will be required), a plasma is created between theelectrodes. Once the plasma has been created, the plasma will affectsthe electrical characteristics of the electrode structure such that itis resonant at the generator output frequency (or frequencies similarthereto), although this resonance will be not be as pronounced becausethe Q of the resonant assembly when a plasma has been created is lowerthan the Q of the strike electrode 260S.

It is not essential that the strike electrode 260S and an “ignited”electrode structure 260, 70 (i.e. the electrode structure 260, 70 with acreated plasma) have identical resonant frequencies in order to benefitfrom this dual electrode ignition technique, merely that they are eachcapable of interacting with the generator output to strike and thenmaintain a plasma without having to retune the generator output.Preferably, however, the resonant frequencies should be the same towithin the output frequency bandwidth of the generator. For example, ifthe generator produced an output of 2450 MHz and at this frequency thisoutput had an inherent bandwidth of 2 MHz, so that, in effect, at thisselected frequency the generator output signal is in the frequency range2449-2451 MHz, the two resonant frequencies should both lie in thisrange for optimum effect.

Referring now to FIG. 18, in a further embodiment which providesindependent ignition of the plasma, an instrument includes a plasmaignition assembly 470S and an electrode structure 470 which areseparately wired (and mutually isolated) to a circulator 414 within theinstrument. Output signals from the generator pass initially into thecirculator 414. The circulator passes the output signals preferentiallyinto the output channel providing the best match to the generator. Aswith the previous embodiment, prior to ignition of a plasma, the matchinto the electrode structure 470 is poor, whereas the ignition assemblyis configured to provide a good match prior to ignition, and so thegenerator output is passed by the circulator into the ignition assembly470S. Since it is wired independently, the ignition assembly 470 may beprovided by any suitable spark or arc generator which is capable ofproducing a spark or arc with power levels available from the generator.For example, the ignition assembly can include a rectifying circuit anda DC spark generator, a resonant assembly to provide voltagemultiplication as in the embodiment of FIG. 16 or any other suitablespark or arc generator. Once ignition of the plasma has occurred, theresultant change in the electrical characteristics of the electrodestructure cause matching of the generator output into the electrodestructure, and so the circulator then acts to divert the generatoroutput into the electrode structure to enable delivery of power into theplasma.

In the majority of embodiments of the surgical system described above anoscillating electric field is created between two electrodes, both ofwhich are substantially electrically isolated from the patient(inevitably there will be an extremely low level of radiation outputfrom the instrument in the direction of the patient, and possibly somebarely detectable stray coupling with the patient), whose presence isirrelevant to the formation of a plasma A plasma is struck between theelectrodes (by the acceleration of free electrons between theelectrodes) and the plasma is expelled from an aperture in theinstrument, primarily under the influence of the pressure of gassupplied to the instrument. As a result, the presence of a patient'sskin has no effect on the formation of a plasma (whereas in the priorart a plasma is struck between an electrode within an instrument and thepatient's skin) and the patient does not form a significant conductivepathway for any electrosurgical currents.

In a particularly preferred instrument best suited to operation with ahigh output power generator such as the above-described generatorembodiments having a magnetron as the output power device, a dualmatching structure such as those included in the instrument embodimentsdescribed above with reference to FIGS. 2 and 14, is not required.Referring to FIGS. 19 and 20, this preferred instrument comprises acontinuous conductive sleeve 50 having its proximal end portion fixedwithin and electrically connected to the outer screen of a standard(N-type) coaxial connector, and an inner needle electrode 54 mounted inan extension 42 of the connector inner conductor. Fitted inside thedistal end portion 70 of the sleeve outer conductor 50 is a heatresistant dielectric tube 180 made of a low loss dielectric materialsuch as quartz. As shown in FIGS. 19 and 20, this tube extends beyondthe distal end of the sleeve 50 and, in addition, extends by a distanceof at least a quarter wavelength (the operating wavelength λ) inside thedistal portion 70. Mounted inside the quartz tube where it is within thedistal end portion 70 of the sleeve 50 is a conductive focusing element480 which may be regarded as a parasitic antenna element for creatingelectric field concentrations between the needle electrode 54 and thedistal end portion 70 of the sleeve 50.

Adjacent the connector 26, the sleeve 50 has a gas inlet 32 and providesan annular gas conduit 38 extending around the inner conductor extension42, the needle electrode 8, and distally to the end of the quartz tube180, the latter forming the instrument nozzle 180N. A sealing ring 482prevents escape of gas from within the conduit 38 into the connector 26.

When connected to a coaxial feeder from a generator such as thatdescribed above with reference to FIG. 8, the proximal portion of theinstrument, comprising the connector 26 and the connector innerconductor extension 42, constitutes a transmission line having acharacteristic impedance which, in this case, is 50 ohms. A PTFE sleeve26S within the connector forms part of the 50 ohms structure.

The needle electrode 54 is made of heat resistant conductor such astungsten and has a diameter such that, in combination with the outersleeve 50, it forms a transmission line section of higher characteristicimpedance than that of the connector 26, typically in the region of 90to 120 ohms. By arranging for the length of the needle electrode, i.e.the distance from the connector inner conductor extension 42 to its tip54T (see FIG. 20), to be in the region of λ/4, it can be made to act asan impedance transformation element raising the voltage at the tip 54Tto a level significantly higher than that seen on the 50 ohms section(inner conductor extension 42). Accordingly, an intense E-field iscreated between the tip 54T of the inner electrode needle and theadjacent outer conductor distal end portion 70. This, in itself, givensufficient input power, can be enough to create a gas plasma extendingdownstream from the tip 54T and through the nozzle 180N. However, morereliable striking of the plasma is achieved due to the presence of thefocusing element 480.

This focussing element 480 is a resonant element dimensioned to have aresonant frequency when in-situ in the quartz tube, in the region of theoperating frequency of the instrument and its associated generator. Aswill be seen from the drawings, particularly by referring to FIG. 20,the resonant element 480 has three portions, i.e. first and secondfolded patch elements 480C, folded into irregular rings dimensioned toengage the inside of the quartz tube 180, and an interconnectingintermediate narrow strip 480L. These components are all formed from asingle piece of conductive material, here spring stainless steel, theresilience of which causes the element to bear against tube 180.

It will be appreciated that the rings 480C, in electrical terms, arepredominately capacitive, whilst the connecting strip 480L ispredominately inductive. The length of the component approaches λ/4.These properties give it a resonant frequency in the region of theoperating frequency and a tendency to concentrate the E-field in theregion of its end portions 480C.

In an alternative embodiment (not shown) the focussing element may be ahelix of circular or polygonal cross section made from, e.g. a springymaterial such as tungsten. Other structures may be used.

The focussing element is positioned so that it partly overlaps theneedle electrode 54 in the axial direction of the instrument and,preferably has one of the regions where it induces high voltage inregistry with the electrode tip 54T.

It will be understood by those skilled in the art that at resonance thevoltage standing wave on the focussing element 480 is of greatestmagnitude in the capacitive regions 480C. The irregular, folded,polygonal shape of the capacitive sections 480C results in substantiallypoint contact between the focussing element and the inner surface of thequartz tube 180. This property, together with the E-field concentratingeffect of the resonator element structure and the presence close by ofthe high dielectric constant material of the inserted tube 180, allserve to maximise the filed intensity, thereby to ensure striking of aplasma in gas flowing through the assembly.

In practice, arcing produced by the focussing element 480 acts as aninitiator for plasma formation in the region surrounding the electrodetip 54T. Once a plasma has formed at the tip 54T it propagates along thetube, mainly due to the flow of gas towards the nozzle 180N. Once thishas happened, the instrument presents an impedance match for thegenerator, and power is transferred to the gas with good efficiency.

One advantage of the focussing element is that its resonant frequency isnot especially critical, thereby simplifying manufacture.

Referring to FIGS. 21 to 24, an instrument 500 for use in the surgicalsystem described above with reference to FIG. 1 comprises twointerconnecting sections, a handpiece 501 and a disposable assembly 502.The instrument 500 comprises a casing 503, closed at the rear by an endcover 504, through which is fed a coaxial cable 505. The centralconductor of the coaxial cable 505 is connected to an inner electrode506, formed of Molybdenum. The outer conductor of the coaxial cable isconnected to an outer electrode 507, shown in FIG. 23. The outerelectrode comprises a hollow base portion 508, with a gas inlet hole 509formed therein, and a tubular extension 510 extending from the baseportion. The inner electrode extends longitudinally within the outerelectrode 507, with dielectric insulators 511 and 512 preventing directelectric contact therebetween.

A gas inlet 513 passes through the end cover 504, and communicates via alumen 514 within the casing, through the gas inlet hole 509 in the outerelectrode, and through further channels 515 in the insulator 512,exiting in the region of the distal end of the inner electrode 506.

The disposable assembly 502 comprises a quartz tube 516, mounted withina casing 517, a silicone rubber washer 518 being located between thecasing and the tube. The casing 517 has a latch mechanism 519 such thatit can be removably attached to the casing 503, via a correspondingdetent member 520. When the disposable assembly 502 is secured to thehandpiece 501, the quartz tube 516 is received within the handpiece,such that the inner electrode 506 extends into the tube, with thetubular extension 510 of the outer electrode 507 extending around theoutside of the tube 516.

A resonator in the form of a helically wound tungsten coil 521 islocated within the tube 516, the coil 521 being positioned such that,when the disposable assembly 502 is secured in position on the handpiece501, the proximal end of the coil is adjacent the distal end of theinner electrode 506. The coil is wound such that it is adjacent and inintimate contact with the inner surface of the quartz tube 516.

In use a gas, such as nitrogen, is fed through the gas inlet 513, andvia lumen 514, hole 509, and channels 519 to emerge adjacent the distalend of the inner electrode 506. A radio frequency voltage is supplied tothe coaxial cable 505, and hence between electrodes 506 and 507. Thecoil 521 is not directly connected to either electrode, but is arrangedsuch that it is resonant at the operating frequency of the radiofrequency voltage supplied thereto. In this way, the coil 521 acts topromote the conversion of the gas into a plasma, which exits from thetube 516 and is directed on to tissue to be treated.

The parameters of the helical coil 521 that affect its resonantfrequency include the diameter of the wire material used to form thecoil, its diameter, pitch and overall length. These parameters arechosen such that the coil has its resonant frequency effectively at theoperating frequency of the signal applied to the electrodes. For a 2.47GHz operating frequency (and a free-space wavelength of approximately121 mm), a resonator coil was employed having an approx coil length of13 mm, a pitch of 5.24 mm, outer diameter 5.43 mm, wire diameter of 0.23mm, and overall wire length of 41.8 mm. This gives a coil with aresonant frequency of approx 2.517 GHz (the difference allowing for thedifferent speeds of propagation of e/m radiation in air and quartzrespectively).

Following repeated use of the instrument, the resonant coil 521 willneed to be replaced on a regular basis. The arrangement described aboveallows for a disposable assembly to be provided which is quick and easyto attach and detach, and also repeatedly provides the accurate locationof the resonant coil 521 with respect to the electrode 506.

Referring to FIG. 25, an electrosurgical system is shown comprising agenerator 4, and an instrument 500 connected to the generator by meansof a cable 12. The instrument 500 comprises a re-usable handpiece 501and a disposable assembly 502, as previously described. The handpiecemay be stored in a holder 532 present on the generator 4 when not inuse.

The disposable assembly is delivered to the user in a sealed packagecontaining the assembly 502 and a button 530. The button 530 is anelectronic key such as that obtained from Dallas Semiconductor Corp andknown by the trademark “i-button”.

When the user attaches a new disposable assembly 502 to the handpiece501, they also connect the button 530 to a reader 531 present on thegenerator 4. The button 530 contains a unique identifying code which isread by the reader 531 and confirmed by the generator 4. If the codecarried by the button is not a recognisable or valid code, the generatordoes not supply pulses of energy to the handpiece. If the code isrecognised as a valid code, the generator supplies pulses of energy tothe handpiece 501. The generator sends information to the button 530concerning the time at which the pulses of energy are first supplied,this information being written to and stored on the button 530.

The pulses of energy supplied by the generator 4 to the handpiece 501can be adjusted by the user so as to be at different energy levelsettings. This adjustment is carried out by means of user interface 533present on the generator 4. The generator sends a signal to the button530 updating an incremental counter every time a series of 10 pulses aresupplied to the handpiece 501. (If there is sufficient memory capabilityin the button 530, the generator can send a signal for each pulse ofenergy supplied). The generator increments the counter by 1 unit foreach pulse supplied the energy of which is less than 2 Joules, and by 2units for each pulse the energy of which is greater than 2 Joules. Theincrements are written to the button 530 and stored thereon. When theincremental counter reaches a maximum value, say 2,400 units, the supplyof energy to the handpiece is halted and a signal is displayedindicating that the disposable assembly 502 must be replaced.

To obtain further treatment pulses, a new pack containing a freshdisposable assembly must be opened, and the new button presented to thereader on the generator. This ensures that the disposable assembly isreplaced before the resonant coil 521 degrades to an unacceptable level.The incrementing of the counter takes into account that the degradationwill occur more quickly with higher energy pulses than with lower energypulses.

It is often the case that the apparatus of FIG. 25 may be used to giverepeated treatment sessions to a particular patient. In this case thebutton 530 is removed from the reader 531, and re-presented at somelater time. The reader 531 will then read from the button 530 the valueof the incremental counter, and calculate whether the incrementalcounter is at its maximum value. If the counter is below the maximumvalue, treatment is allowed to continue.

As the value for the incremental counter is stored on the button 530 asopposed to being stored in the generator 4, the validation will beperformed even if the button 530 is presented to a different generator,say at a different location. This is not the case with various prior artsystems in which all of the information on past usage is stored in thegenerator itself. This prevents the usage-limitation being circumventedby simply taking the button to a different generator, and also allowsthe legitimate situation where a patient may conceivably attenddifferent sites for subsequent treatments.

In addition to preventing the operation of the handpiece once the usagelimit has been reached, the system may also prevent further operation ifthe button 530 is presented to the reader 531 after a period of timefollowing the first use of the disposable assembly 502. As statedpreviously, the time of first use is written to the button 530. If, whenthe button is presented again, a predetermined period of time haselapsed (say 10 hours), then further operation of the handpiece isprevented.

As mentioned above, the use of UHF signals is not essential to theoperation of the present invention, and the invention may be embodied atany frequency from DC signals upwards. However, the use of UHF signalshas an advantage in that components whose length is one quarterwavelength long may be incorporated within compact surgical instrumentsto provide voltage transformation or matching. In addition severalinstruments have been illustrated which have resonant assemblies for thepurpose of step-up voltage transformation, but this is not essential,and upward voltage transformation can be performed within an instrumentwithout making use of resonance.

If the instruments disclosed herein are intended for clinical use, it ispossible to sterilise them, and this may be performed in a number ofways which are known in the art, such as the use of gamma radiation, forexample, or by passing a gas such as ethylene oxide through theinstrument (which will ensure that the conduit for the gas issterilised). The sterilised instruments will then be wrapped in asuitable sterile package which prevents the ingress of contagiontherein.

The various modifications disclosed herein are not limited to theirassociation with the embodiments in connection with which they werefirst described, and may be applicable to all embodiments disclosedherein.

1. A system for controlling the use of a device for treating humantissue, the system comprising: a generator for providing pulses ofenergy to the device, the pulses of energy being at selected energylevels; a controller for controlling the supply of energy to the device;an electronic key associated with the device and including memory means;and a read/write device associated with the controller for downloadinginformation from the electronic key and writing information to thememory means; the controller causing the read/write device to sendsignals to the memory means when energy pulses are provided to thedevice by the generator to cause updating of a device usage counter inthe memory means, whereby the rate at which the counter is incrementedincreases as the delivered power is increased, the controller furthercausing the generator to cease the provision of pulses when the deviceusage counter reaches a predetermined maximum value.
 2. A systemaccording to claim 1, wherein the controller is configured to cause theread/write device to send a counter-incrementing signal to the memorymeans each time a predetermined amount of energy is provided to thedevice by the generator, and wherein the controller updates the counterby a first incremental value for every pulse provided to the devicewhich is below a predetermined energy threshold level, and updates thecounter by a second larger value for every pulse provided to the devicewhich is above the predetermined energy threshold level.
 3. A systemaccording to claim 1, wherein the device is a gas plasma tissuetreatment device.
 4. A system according to claim 2, wherein thepredetermined threshold energy level is in the range of 0.5 to 2.5Joules.
 5. A system according to claim 4, wherein the predeterminedthreshold energy level is substantially 2 Joules.
 6. A system accordingto claim 3, wherein the second value is substantially twice the value ofthe first value.
 7. A system according to claim 3, wherein thepredetermined maximum value is between 500 and 5000 times the firstvalue.
 8. A system according to claim 7, wherein the predeterminedmaximum value is between 2000 and 3000 times the first value.
 9. Asystem according to claim 1, wherein the memory means includes a uniqueidentifying code for the electronic key.
 10. A system according to claim1, wherein the controller is adapted to cause the read/write device tosend a signal to the memory means representative of the time that thekey is first presented to the read/write device.
 11. A system accordingto claim 1, wherein the controller is adapted to cause the read/writedevice to send a signal to the memory means representative of the timethat the first pulses of energy are provided to the device.
 12. A systemaccording to claim 10, wherein the controller is adapted to compare thecurrent time with the representative time, and to prevent the provisionof pulses when the time difference exceeds a predetermined value.
 13. Asystem according to claim 12, wherein the predetermined value is between6 and 12 hours.
 14. A method of controlling the use of a device fortreating human tissue comprising the steps of: providing a controllerfor controlling the supply of energy to the device, and an electronickey associated with each device, the electronic key including memorymeans, presenting the electronic key to the controller, reading thevalue from an incremental counter on the memory means and determiningwhether it has reached a predetermined maximum value, supplying pulsesof energy to the device if the incremental counter has not reached itspredetermined maximum value, and updating the incremental counter at arate which increases as the power provided to the device is increased.15. A method according to claim 14, in which the increase in the rate ofincrementing the counter as the power increases is brought about byupdating the incremental counter by a first value each time that a pulseor a number of pulses is provided to the device below a predeterminedthreshold energy level, and updating the incremental counter by a secondlarger value each time that a pulse or a number of pulses is provided tothe device above the predetermined threshold energy level.